In a manufacturing process of a semiconductor light emitting element, which is formed of GaN (gallium nitride) series compound semiconductor, there is known a technique of a laser lift-off for separating a crystalline layer of a GaN series compound, which is formed on a sapphire base plate, therefrom by irradiation with laser light from a back side of the sapphire base plate. Hereinafter, a laser lift-off refers to separation of such a crystalline layer (hereinafter referred to as a material layer), which is formed on a base plate, from the base plate by irradiating the material layer with laser light. For example, Patent Literature 1 discloses a GaN layer is formed on a sapphire base plate, and GaN, which forms the GaN layer, is broken down by irradiating it with laser light from a back side of the sapphire base plate, so that the GaN layer is separated from the sapphire base plate. A piece, in which the material layer is formed on the base plate, is referred to as a workpiece.
Patent Literature 2 discloses a workpiece is irradiated with linear laser light over a sapphire base plate, while the workpiece is conveyed. Specifically, the literature teaches, as shown in FIG. 12, laser light 124 is formed so that an irradiation region 123 on a boundary face between a material layer 122 of a GaN series compound and a sapphire base plate 121 may be linear, and while moving the sapphire base plate 121 in a direction perpendicular to a longitudinal direction of the laser light 124, the sapphire base plate 121 is irradiated with the laser light 124 from a back side of the sapphire base plate 121. In order to separate the GaN series compound material layer from the sapphire base plate, it becomes important to irradiate the entire face of the workpiece with the laser light, which has irradiation energy required for breaking down the GaN series compound into Ga and N2.